ALBERT

Description: We present dynamic simulations of energy-dependent losses in the radiation belt " slot region" and the formation of the two-belt structure for the quiet days after the March 1st storm. The simulations combine radial diffusion with a realistic scattering model, based data-driven spatially and temporally-resolved whistler mode hiss wave observations from the Van Allen Probes satellites. The simulations reproduce Van Allen Probes observations for all energies and L-shells (2 to 6) including (a) the strong energy-dependence to the radiation belt dynamics (b) an energy-dependent outer boundary to the inner zone that extends to higher L-shells at lower energies and (c) an " S-shaped" energy-dependent inner boundary to the outer zone that results from the competition between diffusive radial transport and losses. We find that the characteristic energy-dependent structure of the radiation belts and slot region is dynamic and can be formed gradually in ~15 days, although the " S-shape" can also be reproduced by assuming equilibrium conditions. The highest energy electrons (E 〉 300 keV) of the inner region of the outer belt (L ~ 4-5) also constantly decay, demonstrating that hiss wave scattering affects the outer belt during times of extended plasmasphere. Through these simulations, we explain the full structure in energy and L-shell of the belts and the slot formation by hiss scattering during storm recovery. We show the power and complexity of looking dynamically at the effects over all energies and L-shells and the need for using data-driven and event-specific conditions.

Description: The quasi-DC compressions of the Earth's dayside magnetic field by ram-pressure fluctuations in the solar wind are characterized by using multiple GOES spacecraft in geosynchronous orbit, multiple Los Alamos spacecraft in geosynchronous orbit, global MHD simulations, and ACE and WIND solar-wind measurements. Owing to the inward-outward advection of plasma as the dayside magnetic field is compressed, magnetic field compressions experienced by the plasma in the dayside magnetosphere are greater than the magnetic-field compressions measured by a spacecraft. Theoretical calculations indicate that the plasma compression can be a factor of 2 higher than the observed magnetic-field compression. The solar-wind ram-pressure changes causing the quasi-DC magnetospheric compressions are mostly owed to rapid changes in the solar-wind number density associated with the crossing of plasma boundaries; an Earth crossing of a plasma boundary produces a sudden change in the dayside magnetic-field strength accompanied by a sudden inward or outward motion of the plasma in the dayside magnetosphere. Superposed epoch analysis of high-speed-stream-driven storms was used to explore solar-wind compressions and stormtime geosynchronous magnetic-field compressions, which are of particular interest for the possible contribution to the energization of the outer electron radiation belt. The occurrence distributions of dayside magnetic-field compressions, solar-wind ram-pressure changes, and dayside radial plasma-flow velocities were investigated: all three quantities approximately obey power-law statistics for large values. The approximate power-law indices for the distributions of magnetic compressions and ram-pressure changes were both -3.

Description: A number of relativistic electron loss processes exist in the inner magnetosphere, and the extent to which MeV electron precipitation into Earth's atmosphere plays a role in radiation belt dynamics is a topic of much debate. In this work, we investigate the contribution of electron precipitation to radiation belt losses, looking at what times and locations precipitation is important. Through high-cadence low-altitude measurements from the SAMPEX satellite, we examine the distributions of millisecond (microburst) as well as longer duration (band-type) precipitation and the relative contributions of these two precipitation types to radiation belt dynamics during high speed stream (HSS) driven storms. Different local time and radial distributions between microbursts and precipitation bands suggest different scattering mechanisms as the causes of the two precipitation types. In a superposed epoch study of 42 HSS-driven storms, enhanced main and recovery phase losses to the atmosphere are observed. Microburst occurrence rates peak in the recovery phase of the storms, while their magnitudes remain fairly constant over storm phase. Precipitation bands show an increase in both occurrence and magnitude at storm onset, particularly at the inner edge of the outer radiation belt. The observations, enabled by the high time resolution and large geometric factor and field of view of the SAMPEX/HILT instrument, reveal when and where microburst and band-type precipitation are contributing to radiation belt dynamics during HSS-driven storms.

Description: We used the inferred equatorial mass density ρ m,eq based on measurements of Alfven wave frequencies measured by the GOES satellites during 1980–1991 in order to construct a number of different models of varying complexity for the equatorial mass density at geostationary orbit. The most complicated models are able to account for 66% of the variance with a typical variation from actual values of a factor of 1.56. The factors that influenced ρ m,eq in the models were, in order of decreasing importance, the F10.7 EUV index, magnetic local time, MLT, the solar wind dynamic pressure P dyn , the phase of the year, and the solar wind B Z (GSM Z direction). During some intervals, some of which were especially geomagnetically quiet, ρ m,eq rose to values that were significantly higher than those predicted by our models. For 10 especially quiet intervals, we examined long-term (〉 1 day) apparent refilling, the increase in ρ m,eq at a fixed location. We found that the behavior of ρ m,eq varies for different events. In some cases, there is significant apparent refilling, whereas in other cases ρ m,eq stays the same or even decreases slightly. Nevertheless, we showed that on average ρ m,eq increases exponentially during quiet intervals. There is variation of apparent refilling with respect to the phase of the solar cycle. On the third day of apparent refilling, ρ m,eq has on average a similar value at solar maximum or solar minimum, but at solar maximum, ρ m,eq begins with a larger value and rises relatively less than at solar minimum.

Description: The outer proton radiation belt (OPRB) and outer electron radiation belt (OERB) at geosynchronous orbit are investigated using a reanalysis of the LANL CPA (Charged Particle Analyzer) 8-satellite 2-solar-cycle energetic-particle data set from 1976-1995. Statistics of the OPRB and the OERB are calculated, including local-time and solar-cycle trends. The number density of the OPRB is about 10 times higher than the OERB, but the 1-MeV proton flux is about 1000 times less than the 1-MeV electron flux because the proton energy spectrum is softer than the electron spectrum. Using a collection of 94 high-speed-stream-driven storms in 1976-1995, the stormtime evolutions of the OPRB and OERB are studied via superposed-epoch analysis. The evolution of the OERB shows the familiar sequence (1) prestorm decay of density and flux, (2) early-storm dropout of density and flux, (3) sudden recovery of density, and (4) steady stormtime heating to high fluxes. The evolution of the OPRB shows a sudden enhancement of density and flux early in the storm. The absence of a proton dropout when there is an electron dropout is noted. The sudden recovery of the density of the OERB and the sudden density enhancement of the OPRB are both associated with the occurrence of a substorm during the early stage of the storm when the superdense plasma sheet produces a “strong-stretching phase” of the storm. These stormtime substorms are seen to inject electrons to 1 MeV and protons to beyond 1 MeV into geosynchronous orbit, directly producing a suddenly enhanced radiation-belt population.

Description: A new empirical model of the electron fluxes and ion fluxes at geosynchronous orbit (GEO) is introduced, based on observations by Los Alamos National Laboratory (LANL) satellites. The model provides flux predictions in the energy range ~1 eV to ~40 keV, as a function of local-time, energy, and the strength of the solar-wind electric field (the negative product of the solar wind speed and the z-component of the magnetic field). Given appropriate upstream solar-wind measurements, the model provides a forecast of the fluxes at GEO with a ~1 hour lead time. Model predictions are tested against in-sample observations from LANL satellites, and also against out-of-sample observations from the CEASE-II detector on the AMC-12 satellite. The model does not reproduce all structure seen in the observations. However, for the intervals studied here (quiet and storm times) the Normalized-Root-Mean-Squared-Deviation (NRMSD) 〈 ~0.3. It is intended that the model will improve forecasting of the spacecraft environment at GEO and also provide improved boundary/input conditions for physical models of the magnetosphere.

Description: The distribution of mass density along the field lines affects the ratios of toroidal (azimuthally oscillating) Alfvén frequencies, and given the ratios of these frequencies we can get information about that distribution. Here we assume the commonlyused power law form for the field line distribution, ρ m  =  ρ m , eq ( LR E / R ) α , where ρ m , eq is the value of the mass density ρ m at the magnetic equator, L is the L shell, R E is the Earth's radius, R is the geocentric distance to a point on the field line, and α is the power law coefficient. Positive values of α indicate that ρ m increases away fromthe magnetic equator, zero value indicates that ρ m is constant along the magnetic field line, and negative α indicates that there is a local peak in ρ m at the magnetic equator. Using 12 years of observations of toroidal Alfvén frequencies by the Geostationary Operational Environmental Satellites (GOES), we study the typical dependence of inferred values of α on the magnetic local time (MLT), the phase of the solar cycle as specified bythe F10.7 extreme ultraviolet solar flux, and geomagnetic activity as specified by the auroral electrojet (AE) index. Over the mostly dayside range of the observations, we find that α decreases with respect to increasing MLT and F10.7, but increases with respect to increasing AE. We develop a formula that depends on all three parameters, α 3Dmodel  = 2.2 + 1.3 ⋅  cos (MLT ⋅ 15 ∘ ) + 0.0026 ⋅ AE ⋅  cos ((MLT − 0.8) ⋅ 15 ∘ ) + 2.1 ⋅ 10 − 5  ⋅ AE ⋅ F10.7 − 0.010 ⋅ F10.7, that models the binned values of α within a standard deviation of 0.3. While we do not yet have a complete theoretical understanding of why α should depend on these parameters in such a way, we do make some observations and speculations about the causes. At least part of the dependence is related to that of ρ m , eq ; higher α , corresponding to steeper variation with respect to MLAT, occurs when ρ m , eq is lower.

Description: We simulate whistler mode waves using a hybrid code. There are four species in the simulations, hot electrons initialized with a bi-Maxwellian distribution with temperature in the direction perpendicular to background magnetic field greater than that in the parallel direction, warm isotropic electrons, cold inertialess fluid electrons and protons as an immobile background. The density of the hot population is a small fraction of the total plasma density. Comparison between the dispersion relation of our model and other dispersion relations shows that our model is more accurate for lower frequency whistlers than for higher frequency whistlers. Simulations in 2-D Cartesian coordinates agree very well with those using a full dynamics code. In the 1-D simulations along the dipole magnetic field, the predicted frequency and wave number are observed. Rising tones are observed in the 1/14 scale simulations that have larger than realistic magnetic field spatial inhomogeneity. However, in the full scale 1-D simulation in a dipole field, the waves are more broadband and do not exhibit rising tones. In the 2-D simulations in a meridional plane, the waves are generated with propagation approximately parallel to the background magnetic field. However, the wave fronts become oblique as they propagate to higher latitudes. Simulations with different plasma density profiles across L -shell are performed to study the effect of the background density on whistler propagation.

Description: The Naval Research Laboratory SAMI3 (Sami3 is Also a Model of the Ionosphere) and the RAM–CPL (Ring current Atmosphere interaction Model–Cold PLasma) codes are used to model observed plasmasphere dynamics during 2001 November 25–December 1 and 2001 February 1–5. Model results compare well to plasmasphere observations of electron and mass densities. Comparison of model results to refilling data and to each other shows good agreement, generally within a factor of 2. We find that SAMI3 plasmaspheric refilling rates and ion densities are sensitive to the composition and temperature of the thermosphere and exosphere, and to photoelectron heating. Results also support our previous finding that the wind-driven dynamo significantly impacts both refilling rates and plasmasphere dynamics during quiet periods.

Description: Abstract An existing empirical model of the electron fluxes at geosynchronous orbit is extended radially outward in the equatorial plane to ~6–20 Earth radii (RE) using observations from the Research with Adaptive Particle Imaging Detectors (RAPID) instrument on the Cluster spacecraft. The new model provides electron flux predictions in the energy range ~45 eV to ~325 keV, as a function of local time and radial distance from the Earth, with geomagnetic activity parameterized by the Kp index. The model outputs include the mean and median electron fluxes along with the standard deviation and the 5th, 25th, 75th, and 95th percentiles for the given input conditions. The flux outputs from the model are tested against in‐sample observations from Cluster/RAPID and out‐of‐sample observations from Time History of Events and Macroscale Interactions during Substorms (THEMIS)/Solid State Telescope with good prediction efficiency during quiet and active intervals, as quantified by standard methods. This new model is intended to supplement current predictive capabilities in the magnetosphere for spacecraft operations, as well as providing the necessary boundary and/or input conditions for computational/physical models of the magnetospheric system when the necessary in situ observations are unavailable. While the new model can certainly not reproduce the rapid small‐scale fluctuations inherent in spacecraft observations, it does provide a coarse capability to predict the flux of electrons close to the equatorial plane, based on radial distance, energy, local time, and geomagnetic activity, in regions where no in situ assets are available.